Digital computers employing microprocessors as their central processing unit are commonly employed as what are termed "personal computers" or "engineering work stations." These self-contained devices will hereinafter be generically referred to as "personal computers." In addition to their central processing units, formed on a single integrated circuit chip, they include semiconductor random access memory, semiconductor read only memory for storing their operating system, some form of mass memory, usually in magnetic form, a keyboard for introducing user-operated commands, and a self-contained power supply.
In a popular form of personal computer, modeled after the IBM PC line, these circuits are all formed on or interconnected to a single large printed circuit board, termed the "mother board". The mother board includes female connectors of the type adapted to receive edge connectors formed on printed circuit boards, termed the "system expansion slot," which are electrically interconnected to the address and data bus of the personal computer. Through use of the system's expansion slot and associated mechanical elements within the housing for the personal computer, the personal computer may be expanded by adding printed circuit boards containing specialized electronic circuits so that they interconnect directly with the personal computer.
Personal computers are used for two broad purposes: self-contained processing of data, including text, and communication with remote data sources and sinks. Some personal computers are dedicated solely to self-contained processing activities such as accounting functions or word processing using text introduced by a keyboard or magnetic disks, and require no communication with remote stations. Other personal computers are used for communications such as electronic mail terminals, data collection devices, terminals for remote computers or data bases, and the like. Often a personal computer will be dedicated to one of these tasks but in other situations such dedication is not economical and it is preferable for a personal computer to perform a variety of mixed local processing and communication tasks often on an overlapping time basis.
Without the use of highly specialized software, a personal computer that is used to receive communications that occur on a nonpredictable time schedule, cannot be used simultaneously for local processing tasks. Rather, the computer must be dedicated to the receiving operation so that it is always in the proper status to receive communications. Since many communication operations utilize only a small fraction of the receiver's time, this is often a uneconomic commitment of computer resources.